A Patterson-like scheme is proposed for direct inversion of the conventional low-energy electron diffraction (LEED) intensity versus energy I(E) curves, which is in contrast with the previously suggested holographic scheme. Using the Si(111)-(7 X 7) and Si(113)-(3 X 2) surfaces as examples, high quality three-dimensional images, with a resolution better than 0.5 Angstrom, of both surface atoms and bulk atoms are obtained from the direct Patterson inversion of LEED-I(E) curves with the integral-energy phase-summing method.

Several long-range superstructures have been observed with the scanning tunneling microscopy on the reconstructed Pt(100) surface at room temperature. They are present in strained domains and involve both the Fermi electrons and the concomitant lattice distortions. A first-principles calculation shows that the top layer expanded similar to 18% on average and the Fermi surface for a single hexagon layer displays some nesting portions, which can be related to the wave vectors of the observed superstructures. Thus, these superstructures existing in the local domains of the reconstructed surface have the likely origin of incipent charge density waves.

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Trimer is the smallest cluster that can have a one-dimensional or a two-dimensional structure on fee (111) surface. Using first-principles density-functional-theory calculations, the structural and dynamical properties of Al trimer on Al(111) surface have been studied in detail. Al trimer on Al(111) surface has four close-packed (compact) triangular configurations, two linear configurations, and some other noncompact triangular configurations. The close-packed triangular trimers are more stable than the noncompact triangular trimers as well as the linear trimers. For the dynamics of Al trimer on Al(111) surface, the diffusion processes are much more complicated than the adatom and dimer diffusions. There are three different kinds of diffusion mechanisms: concerted translations and rotation of compact triangular trimers (the energy barrier

The adatoms, dimers and rest atoms in the three outermost atomic layers of the Si(111)-(7 x 7) surface are directly imaged with glancing Kikuchi electron holography. The applicability of Kikuchi electron holography to complicated multiple-emitter surfaces is evident. The three-dimensional relative positions of atoms on the Si(111)-(7 x 7) surface are in good accordance with the LEED-optimized DAS model.

More than 50 symmetry-inequivalent emitter-scatterer (E-S) pairs were observed for the Si(111)-(7 x 7) surface using Kikchi electron holography (KEH) with various incident/detection configurations. For different configurations, the E-S pairs in the backscattering direction are preferentially enhanced. Thus, one can obtain detailed structural information by changing the incident/detection direction, which will be helpful in sorting out the correct surface structure model.

Atomic structures of the reconstructed Si(113) surfaces were studied by using Kikuchi electron holography (KEH). Three-dimensional images show clearly the characteristics of the puckering model for both Si(113)(3x2) and (3x1) surfaces. The KEH results support the puckering model. Based on bur studies, the tetramers are puckering alternatively in the (3x2) surface. Whereas in (3X1) structures, there are two domains, within each of them, tetramers buckled uniformly, but the overall directions are opposite. When doped with H atoms on a (3x2) surface, the asymmetric tetramers change into symmetric form. [S0163-1829(99)51116-8].

Photoelectron-diffraction studies of Nb(001) have been performed to determine the first-interlayer contraction using Mg K alpha radiation (h nu = 1253.6 eV) as an excitation source. Photoemission intensities of the 3d(5/2) core level were measured as a function of the polar angle along several azimuths on the single-crystal surface. The 202.3-eV (205.0-eV) binding energy for the 3d(5/2) (3d(3/2)) core level was well resolved in the photoemission spectra, where the peak intensity could be easily evaluated by curve-fitting processes. Large oscillations of the 3d(5/2) intensity as a function of the polar angle due to forward-focusing were observed. Based on multiple-scattering calculations for several first-interlayer spacings ranging from the bulk value to 16% contraction, the best agreement with experiment was obtained for a (13 +/- 5)% contraction of the first-interlayer spacing.

When disilane (Si2H6) is used in the homoepitaxial growth of Si by chemical vapor deposition (CVD), the fragment SiH2 is believed to be the basic unit adsorbed on the surface. The bonding site of SiH2 on Si(100) has been proposed in the literature to be either on top of a dimer (the on-dimer site) or between two dimers in the same row (the intrarow site). Since the pathway of SiH2 combination is dependent on the adsorption site, a first-principles calculation will shed light on the underlying process. We have performed self-consistent pseudopotential density-functional calculations within the local-density approximation. On the bare Si(100) surface, the on-dimer site is found to be more stable than the intrarow site, even though the former has unfavorable Si-Si bond angles. This is ascribed to the extra dangling bond created in the latter geometry when the weak dimer a bonds are broken. However, the presence of hydrogen adatoms eliminates this difference and makes the intrarow site more favorable than the on-dimer site. It is therefore revealed in this theoretical study that hydrogen, an impurity unavoidable in the CVD process, plays an important role in determining the stable configuration of adsorbed SiH2 on Si(100) and hence affects the growth mechanism. [S0163-1829(98)52544-1].

Using classical mechanical and quantum Monte Carlo methods we trace the ground-state behavior with an applied magnetic field of localized electron pair states in a quantum dot. By developing a method to treat nonconserved paramagnetic interactions using variational and diffusion quantum Monte Carlo techniques we find (i) a single-triplet transition at very small magnetic field strengths, (ii) enhanced localization of the two electrons with increasing magnetic field, and (iii) a mechanism for pair breakup that is different from that proposed recently by Wan et al. [Phys. Rev. Lett. 75, 2879 (1995)]. [S0163-1829(98)04016-8].